An electron's position is given by r = 4.00t i - 6.00t2 j + 5.00 k, with t in seconds and r in meters.

Question

(a) In unit-vector notation, what is the electron's velocity v(t)? (Answer in terms of i, j, k, and t.)
___ m/s

(b) What is v in unit-vector notation at t = 8.00 s?
___ m/s

(c) What is the magnitude of v at t = 8.00 s?
___ m/s

(d) What angle does v make with the positive direction of the x axis at t = 8.00 s?
___ ° (from the +x axis)

..for (a), i got:

((d(4.00))/dt)i-((d(6.00))/dt^2)j+((d(5.00))/dt)k

and for (b):

((d(4.00))/(d(8.00)))i-((d(6.00))/(d(8.00^2)))j+((d(5.00))/(d(8.00)))k

what am i doing wrong?